In US 2005/0157515 A1, a light emitting diode light source is disclosed having a printed circuit board with a plurality of side faces, a plurality of RGB LED units arranged on one side face of the printed circuit board and each unit having a red LED, a green LED and a blue LED, and at least one control unit connected to each LED in the RGB LED units and controlling a driving current to the LED. Each of the RGB LED units emits white light with a stable color temperature. The control unit has a memory for storing a driving current data for each LED. Whereas, it is described that due to variations in the LED fabrication process a calibration process can be executed on each LED, and the characteristic data assembled during the calibration process can be stored in the memory of the control unit in order to control the color temperature of the LEDs, the problems caused by the LEDs themselves during operation are not recognized. Especially the heat produced by the LEDs strongly influences the operation of the LED.